Soil compaction below optimum moisture

[jpier99463]

I am working on a project where I have to compact my soil to 95% of a modified proctor within a moisture range of + or - 1.5%. In my case this works out to 5% to 8%. I am having trouble increasing the moisture percentage to above 5%. Besides having to perform more work to achieve compaction, what is the harm of achieving compaction this far below the optimum moisture %.

 

[BigH]

Can you please advise what type of soil you are compacting. With the values used it appears to be a sandy gravel - but please confirm.

 

[jpier99463]

BigH: Thank you for responding to my query. I am actually building a military base near Jalalabad, Afghanistan. I would describe the soil as very granular with a lot of rocks in it. The value of the modified proctor came out at 224.5 kg/meter cubed at a moisture content of 6.5%. In the field, we have not been able to achieve moisture content over 5%. I am certainly open to suggestions as to how to increase moisture content. We have been able to achieve compactions between 95 and 103% even under these relatively dry conditions.

 

[GoldDredger]

Here is a quote from my Civil Engineering Reference Manual:

"Soils close to the optimum water content require less compactive effort to achieve the required relative compaction"

In other words, achieving optimum moisture content should be considered a tool to assist the effort of compaction.

If you are achieving 95 to 103 percent compaction I would say you've met your requirements, irregardless of how your moisture content percentage is reading.

Also you mentioned lots of rocks. Be careful in sandy/rocky soils with the large cobbles (6 to 8 inches plus). Many specifications call for the screening out of such rocks because it is difficult to really compact around them (they create litte void spaces, especially when in contact with eachother).

Anyway, sandy soil is usually the best material as a sub base, as opposed to say clay, which can swell or consolodate with added moisture or load respectively.

Good luck, and thank you for your efforts in that part of the world.

 

[Ron]

Agree with GD...other than lost efficiency, compaction is compaction. Go with it.

 

[BigH]

I would compact, for base course or subbase, dry of optimum anyway. The compaction equipment you are using is of higher energy than the Modified Proctor energy anyway so that the optimum moisture content for your equipment would actually be less than in the laboratory test. I would have no problems compacting say at a moisture content of 4% or slightly over. You will get the compaction. The reason I queried the type of material you were compacting was that in some cases (say clay core of a dam) you would want to compact on the wet side of optimum. Agreed that the big stones may cause problems - like segregation. Best if you could screen them out.

 

[Boondog]

I also agree with BigH. We normally use 0 to -2% for sands in the middle east. Moisture control of sands with less than 10% fines can be difficult in the arid climate. Adequate density is easily achieved in a slightly dry condition.

 

[jpier99463]

I appreciate everyone's comments. They are in agreement with my feelings on the subject. While moisture above optimum might pose a problem if you experience pumping and the soil never really compacts and I don't recommend working in these conditions. However, as long as you can achieve the dry density specified even if the moisture is belwo optimum, the soil should be able to handle the load it is designed to carry. I appreciate everyone's input to help me bolster my case if I am questioned by the USACE